Medical implantable devices for neurostimulation and/or detection are becoming increasingly commonplace as manufacturing processes allow for such devices to be produced with small enough form factors to facilitate implantation. Such devices may for instance comprise conductive tracks wound tightly around a carrier, e.g. a lead body or the like. The conductive tracks are for example required to relay signals between a sensor or electrode arrangement within the body and processing circuitry outside the body. The implantable part of the device needs to have certain dimensions, which for instance may depend on the particular implementation. One specific example is for deep brain stimulation, and another example is for cochlear implants.
For example, the cochlea of the human ear contains hair cells that are essential to the perception of sound. Sound vibrations distort certain structures of the cochlea which in turn distort the hair cells. This initiates electrical impulses in the hair cells which are conveyed to the fibers of the auditory nerve and ultimately to the brain.
Some instances of human hearing loss are attributed to extensive destruction of the hair cells. When this occurs, though the structures of the cochlea may otherwise be substantially intact, and the auditory nerve may be partially or completely intact, the auditory response is significantly impaired or non-existent.
Cochlear implants directly stimulate the auditory nerves inside the inner ear. In a traditional cochlear implant system, a microphone acquires sound from the environment. The sound is then selectively filtered by a speech processor, using various filter bank strategies such as Fast Fourier Transforms, to divide the signal into different frequency bands. Once processed, the signal is then sent to a transmitter, a coil held in position by a magnet placed behind the external ear. This transmitter sends the processed signal to the internal device by electromagnetic induction.
Embedded in the skull, behind the ear is a receiver which converts the signal into electric impulses and sends them through an internal cable to electrodes. Conventional cochlear implants are made of multiple platinum electrodes or similar conductive material, connected to platinum wire and embedded in a silicone body. These electrodes then act to stimulate the auditory nerve fibers by generating an electric field when the electrical current is routed to them.
The implant should have a small insertion area so that the installation of the cochlear implant does not damage the fine cochlear structures in the ear. Implants for deep brain stimulation or other nerve stimulation may or have similar constraints on the cable dimensions and insertion area. One known design is based on a long strip of electrodes, which are then wound around a carrier to form a spiral strip cochlear implant. This provides the desired tubular shape for insertion into the cochlea. An example of this type of arrangement is disclosed in US 2012/0310258.
A typical viable manufacturing process of such a strip is shown in FIG. 1. The process commences in step (a) with the provision of a silicon substrate 10 onto which a ceramic dielectric layer or layer stack 12 is deposited using plasma-enhanced chemical vapour deposition (PECVD). Such a ceramic dielectric layer typically comprises SiOx (x>1) and optionally further comprises a silicon nitride layer (Si3N4). In step (c) metal electrodes 14 are formed on the ceramic dielectric layer (stack) 12, e.g. through deposition and patterning of one or more metal layers.
A further ceramic dielectric layer (stack) 16 is deposited using PECVD in step (d), which may be made of the same dielectric material(s) as the ceramic dielectric layer (stack) 12, such that the electrodes 14 are encapsulated by the ceramic dielectric layer (stack) 12 and the further ceramic dielectric layer (stack) 16, after which in step (e) a biocompatible electrically insulating polymer layer 18 such as a parylene layer is coated onto the further ceramic dielectric layer (stack) 16, e.g. through spin-coating or dip-coating. Next, a glass transfer substrate 20 is glued to the biocompatible electrically insulating polymer layer 18 using adhesive 22 in step (f), followed by the removal of the silicon substrate 10, e.g. through etching or by the removal of a sacrificial release layer (not shown) in between the silicon substrate 10 and the ceramic dielectric layer (stack) 12 in step (g). At this stage, the ceramic dielectric layer or layer stack 12 may be patterned to form trenches 13 providing access to the electrodes 14.
In step (h), a further biocompatible electrically insulating polymer layer 24 such as another parylene layer is deposited, e.g. through spin-coating or dip-coating, on the ceramic dielectric layer (stack) 12 such that the strip is embedded in the biocompatible electrically insulating polymer layer 18 and the further biocompatible electrically insulating polymer layer 24. At this stage the further biocompatible electrically insulating polymer layer 24 may be patterned to form trenches 25 providing access to the electrodes 14 through trenches 13. The strip is finalized in step (i) by the removal of the glass transfer substrate 20, e.g. by dissolving the adhesive 22.
This process involves both ceramic and polymer processing steps, which typically require different temperature budgets. Specifically, the PECVD steps are typically performed at a temperature in excess of 120° C. using tetraethyl orthosilicate (TEOS) as a silicon oxide precursor, at which temperatures most biocompatible electrically insulating polymers such as parylene degrade or even decompose.
EP 2 626 110 A1 discloses a thin film for a lead for brain applications with at least one section comprising a high conductive metal and a low conductive metal, whereby the low conductive metal at least partially encapsulates the high conductive metal and is biocompatible.